Mark VanGessel, Extension Weed Specialist; mjv@udel.edu
If you have wanted to kill your cover crops before now and have not had a chance, you may want to evaluate the stand and consider letting it go a little longer. The benefits of cover crops can increase the longer it is allowed to grow, however there are a few things to consider. If it’s not a very thick stand and there are a lot of weeds in the field, some of the weed species may be more difficult to kill if they are allowed to mature. If the stand is thick and you are considering if you should let it grow longer, you have to decide if your planter can cut through the cover crop, properly place the seed, and close the seed furrow. You will also have to evaluate your cover crop in light of altering your fertility program. If you have a lot of rye (or all rye) the cover crop could tie up a lot of your nitrogen. For soybeans that may not be an issue, but for non-legumes the crop may not have as much available nitrogen as it needs. You may need to apply additional nitrogen.
Allowing the cover crop to grow longer will:
- allow roots to grow deeper and wider, thus reducing compaction in the soils
- the additional root growth may mellow the soil and allow for easier planting
- improve soil moisture retention, by reducing evaporation; and the more biomass the better moisture retention
- the greater the cover crop biomass, the better weed suppression it provides (and the longer into the season the residue can provide weed suppression)
Cover crops are growing rapidly at this time of year and the amount of biomass can change dramatically in 7 to 10 days. I recommend planting into a dead cover crop. Planting into a sprayed cover crop before it has completely died can be troublesome, so don’t rush it and allow the cover crop to die. I know some growers are planting their grain crops and then spraying after planting, but this strategy can be challenging. I would not recommend planting into a living cover crop the first time someone decides to allow their cover crops to grow longer.
Cereal cover crops (rye, wheat, barley, triticale) have been controlled consistently with glyphosate. But legumes and brassica species are often not killed with glyphosate and they require special consideration for killing them. Thus, allowing time for the cover crop to die before planting is also a safety net to ensure your burndown sprays will kill them. Otherwise, an additional strategy maybe need to successfully kill the cover.
The weather this spring has not been good for spraying and so there may be a chance to experiment with allowing your cover crop to grow a little longer. Just be sure to consider all the factors and start with an area that is manageable.